This invention relates to the treatment of gas streams comprising hydrogen sulphide.
Several industrial process, particularly in the refining of oil and natural gas, produce waste gas streams that include hydrogen sulphide. Since hydrogen sulphide is particularly poisonous it is necessary to treat such streams so as to extract their sulphur values upstream of their being vented to the atmosphere. One known process for treating a gas stream including hydrogen sulphide is the Claus process. In the Claus process approximately one third of the hydrogen sulphide content of the gas stream is burnt in a furnace to form sulphur dioxide and water vapour. The sulphur dioxide then reacts in the furnace with residual hydrogen sulphide to form sulphur vapour and water vapour. The stoichiometry of these reactions is shown in the following equations: EQU 2H.sub.2 S+30.sub.2 .revreaction.2H.sub.2 O+2SO.sub.2 EQU 2H.sub.2 S+SO.sub.2 .revreaction.2H.sub.2 O+3S
The resulting sulphur vapour tends to exist in a number of different molecular species at different temperatures. Above 800.degree. C., for example, it exists mainly as the dimer S.sub.2. In addition to these reactions, there is a tendency for hydrogen sulphide to dissociate at elevated temperatures into hydrogen and sulphur vapour. This reaction is reversible and on cooling most of the hydrogen and sulphur vapour reassociates to form hydrogen sulphide. Moreover, if carbon dioxide and hydrocarbons are present in the gas stream, which typically occurs if the source of the gas stream is an oil refinery, small amounts of carbonyl sulphide and carbon disulphide are also formed.
The reaction between hydrogen sulphide and sulphur dioxide does not reach completion at the temperatures that are created in the furnace. Indeed, it typically reaches only about 60 to 70% of completion in the furnace. It is therefore the commercial practice to cool the resulting gas stream downstream of the furnace in, for example, a waste heat boiler, then to condense sulphur out of the cooled gas mixture, next to reheat the gas stream to a temperature in the order of 200.degree. to 260.degree. C., and to pass the reheated gas stream over a catalyst, for example alumina, of the reaction between hydrogen sulphide and sulphur dioxide so as to form further sulphur vapour and water vapour. The resulting sulphur vapour is then condensed. With two or three such trains of catalytic stages, it is typically possible to achieve only about 97% conversion of the hydrogen sulphide in the original gas stream. Further such catalytic stages are not normally employed since the concentration of hydrogen sulphide and sulphur dioxide in the gas stream becomes progressively lower with each catalytic stage, thereby adding to the difficulty of obtaining an adequate degree of conversion in each catalytic stage. Increasingly, rigorous standards concerning the protection of the environment make simple venting or incineration of the final gas stream an unattractive or impermissible choice. It is therefore becoming increasingly the commercial practice to pass the final gas stream to a so-called `tail gas clean up` unit which is able effectively to treat the hydrogen sulphide and sulphur dioxide components of the gas stream notwithstanding their low concentrations. There are a number of different `tail gas clean up` processes that are commercially available, for example, the SCOT process.
The Claus process has in recent years excited the interest of suppliers of oxygen separated from air. Conventionally, air had been used to support the combustion of hydrogen sulphide in the furnace. In consequence, large volumes of nitrogen are introduced in the air and flow through each stage of the process. The nitrogen takes up reactor space. It has therefore been proposed to substitute commercially pure oxygen for some or all of the air that is used to support combustion of the hydrogen sulphide, and thereby gain an increase in the rate of which a hydrogen sulphide containing feed gas can be accepted by the furnace. Depending on the exact composition of the feed gas, there can however be a limitation on the degree to which oxygen can be used to enrich the combustion air in oxygen, this limitation being that the temperature created at the lining of the furnace by the combustion increases with increasing concentration of oxygen in the combustion air until a temperature so high that the lining would be damaged in creation. This temperature limitation has been believed to prevent the complete substitution of pure oxygen for the combustion air when a feed stream relatively concentrated in combustibles, say containing more than about 70% by volume of hydrogen sulphide, is treated, although it is now understood that dissociation of hydrogen sulphide (which tends to take place at a higher rate with increasing flame temperature) has a moderating effect and may allow operation with pure oxygen in some circumstances with some configurations of burner and furnace. A number of proposals have been made to alter the Claus process so as to facilitate its use of pure oxygen or oxygen-enriched air to support combustion of hydrogen sulphide. Some of these proposals have involved the introduction of temperature moderating media into the hydrogen sulphide combustion region, sometimes by recycle of gas from a downstream stage of the treatment process, as is disclosed in, for example, EP-A-165 609. Others of these proposals have involved performing the combustion of the hydrogen sulphide in two or more stages, as is disclosed in, for example, EP-A-237 216 and EP-A-237 217.
There remains, however, a need to improve the effective conversion efficiency of a Claus process so as to facilitate downstream treatment of the resulting gas stream. It is an aim of the present invention to provide a method and apparatus that meet this need.